Two insulin-like growth factors (IGF-I and IGF-II) are presently known to exist and to be required for the proliferation of various cells. For example, the topical use of IGF-II for wound-healing is taught in U.S. Pat. No. 4,885,163 (Dec. 5, 1989). It is also reported that IGFs have a particular effect upon the growth of cells of mesodermal origin and on their differentiation, and further, that the IGFs exhibit potency in stimulating DNA synthesis in human fibroblasts and in rat osteoblasts. In addition, it is suggested that IGF-I may serve to stimulate collagen synthesis in human fibroblasts, whereas studies report that IGF-II may have a predominant role in undifferentiated cell proliferation.
Several proteins have been discovered which bind to these IGFs and modulate IGF actions either in an inhibitory or a stimulatory manner, and these proteins are termed insulin-like growth factor binding proteins (IGFBPs).
Insulin-like growth factors (IGF-I and IGF-II) are synthesized by multiple tissues and circulate in plasma to modulate the growth of various cell types. They do not exist in the blood as free hormones but are bound to carriers in the form of IGFBPs. To date three distinct classes of IGFBPs have been characterized, based on their complete primary structure having been obtained by molecular cloning, all of which are able to bind both IGF-I and IGF-II and to modulate IGF actions either in an inhibitory or a stimulatory manner.
Based on the recommendation proposed by an IGFBP conference held in Vancouver, Canada in June 1989, the first BP class whose complete primary structure was deduced has been named IGFBP-1; its structure was deduced from cDNA clones identified in the libraries prepared front a human HEP-G2 hepatoma cell line, from human placenta and from both human and rat decidua. A human genomic clone encoding IGFBP-1 was also isolated and characterized (Brinkman, A., et al., B.B.R.C. 157, 898-907 (1989)), the gene locus of which is mapped at location p12-p13 on chromosome 7, Alitalo, T. et al., Hum. Genet. 83, 335-338 (1989). This protein exhibits a molecular weight (M.sub.r) of 28 kDa on SDS/PAGE under non-reducing conditions and has almost equal binding affinity for IGF-I and IGF-II. It contains no potential N-linked glycosylation sites, but it has at least five potential O-linked glycosylation sites, which may account for a reported 4.3% carbohydrate content of the protein. The circulating level of the IGFBP-1 is elevated in patients and animals with insulin-dependent diabetes mellitus.
The second BP class is one for which the complete primary structure was deduced from cDNAs isolated from a rat BRL-3A cell library as well as from adult rat and human fetal liver libraries; it has been named IGFBP-2. This BP, having M.sub.r of 31.5-33.0 kDa on SDS/PAGE under non-reducing conditions, exhibits equal affinity for IGF-I and IGF-II when IGF-I is used as a radioligand, but it shows a marked preference for IGF-II when the radioligand is IGF-II. The level of IGFBP-2 in rat serum is high in fetus but decreases in adult. The physiological role of IGFBP-2 is not well known, but Adashi et al. Endocrinologqy, 126, 1305-1307 (1990) recently reported that pituitary follicle-stimulating hormone (FSH) inhibits the constitutive release of IGFBP-2 from rat ovarian granulosa cells.
The third BP class is a high molecular weight IGFBP within the 150 kDa IGF-binding complex found in plasma. Its complete primary structure was deduced from human, porcine and rat cDNAs, and it has been named IGFBP-3, see Shimasaki, S., et al., B.B.R.C., 165, 907-912 (1989). This 150 kDa complex consists of three components, an IGFBP-3 of 53 kDa bound to an IGF and an acid-labile 80 kDa protein which can only bind to IGFBP-3 in association with IGF under neutral conditions. Both the 53 kDa IGFBP-3 and the 80 kDa acid-labile subunits are glycosylated. Moreover, the circulating level of the complex is dependent on growth hormone (GH). This protein has recently been isolated from ovarian follicular fluid, and it appears to act as an inhibitor to the FSH-stimulated production of estradiol in cultures of rat ovarian granulosa cells.
Besides these three IGFBPs, it is believed that other BPs for IGF exist.